This may not actually be a chemistry question.
In: Chemistry
There is a certain amount of degradation that may happen while it sits on a shelf. They add things to the medicine to try to keep it as stable as possible. There are regulations about how much a medication can be reduced over time and still be effective. In some cases, like some OTC painkillers, it won’t be that big of a deal, but in others, like expired insulin or epi-pens, it can mean life or death.
Some drugs might get a bit less effective after their use-by date, but in the vast majority of cases the decrease in potency is insignificant. A few drugs might also develop harmful side effects and bacterial growth on them if you keep them for a long time past their use-by date.
First, those dates are very conservative guesses by the manufacturer. The issue is the twin dangers of moisture and oxygen.
Oxygen is a very reactive gas, it will bond to just about anything, creating what are called oxides. Once the oxygen has been bound up with something, it tends to be very difficult or energy intensive to break it free again.
Most drugs we use are rather long chemical chains. And what *shape* that chain takes is often just as important as what molecules or atoms go into making it. (I will avoid digressing on the fascinating subject of drug chirality. )
When oxygen from the air contacts the chemicals in the drugs, it starts binding with those chemicals in a haphazard way. Sometimes it just gets tacked onto the ends, sometimes it actually breaks the chemical bonds because it is more “grabby” than one of the molecules in the original bond. Think of someone holding one of your hands and I come along and push them aside to hold both of your hands. Something similar happens with chemical bonds. When this happens, not only is there less of the original chemical compound available to act as medicine, but the chemicals that got bound by oxygen might become toxic.
One common example is when common aspirin; acetylsalicylic acid, expires. The chemical breaks down into acetic acid (commonly known as vinegar) and salicylic acid. Salicylic acid is the actual active ingredient your body absorbs when you take aspirin, but it is more irritating to the stomach and breaks down more during digestion.
The result is that aspirin with a vinegary smell is going to upset your stomach more and work far less effectively.
Drugs companies can’t predict every change that will occur to their medications, especially when it will be stored and handled by factors beyond their control. So they put very conservative estimates on the bottles. Many decades ago, the USAF did a study as part of nuclear war planning and found that most of the drugs you’d keep in a fall out shelter typically lasted up to 2 times as long as the expiry date would suggest and that the chief effect of age was diminished effectiveness. The ones that had the worst longevity were antibiotics and tricyclics.
Prescription medications, like food, are required to have a use by date. This is only the date that the seller can guarantee (typically in medicine a “guarentee” means with 99.9% assurance or higher) that the medicine will still be safe and effective by that date. While most medication doesn’t go bad by that date, there could be problems such as a reduction in potency. I can’t think if any situations in which a medication would break down into something harmful, unless it is a refrigerated or otherwise sensitive medication.
The bigger problem is typically the introduction of water that may cause bacteria to grow on our in the medication, which isn’t likely but is possible.
Most every chemical will degrade at some point, but it depends on the chemical and method of storage.
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